The folded dipole has several interesting features.
A two wire folded dipole can increase the characteristic feed impedance of a dipole and offer a good match to 300 Ohm balanced feed line.
A three wire folded dipole can increase the characteristic impedance of a dipole and offer a good match to 450 Ohm or 600 Ohm balanced feed line.
Offers a better match over a wider band, which can be important on the lower frequency bands.
When fed with a balanced feed line, and an antenna tuner, it can be run on multiple bands. This assumes that it is 1/2 wavelength long at your lowest operating frequency.
The drawing below shows the essential elements of a folded dipole. It consists of two parallel elements having a constant spacing s. These elements can be anything from simple wires to copper or aluminum tubing. The bottom element is split in the center and serves as the feedpoint. The upper element has a diameter d2 and the bottom element has a diameter d1. The ends of the elements are connected to form a continuous loop from the feedpoint.
The relationship of those three dimensions, (s, d1, and d2) creates a impedance transformation at the feedpoint that is described by the equation on the right. The Ratio, when multiplied by the standard dipole feed impedance, describes the folded dipole feed impedance.
In the text areas to the right, enter your initial design information. Enter your expected frequency of operation, the antenna velocity factor, and the nominal feed impedance of a simple dipole.
The velocity factor is to adjust for the fact that the propogation of energy in a wire is a little slower than in free space. The value is based on the length to diameter ratio and defaults here to 0.951. Larger diameters may require you to adjust this value slightly higher.
The folded dipole multiplies the normal feed impedance of a simple dipole. For a 1/2 wave dipole, in free space, this is approximately 72 Ohms. You may not be dealing with a dipole in free space, but 72 Ohms is close enough to start with. You can adjust it to other impedances in the appropriate text box below.
Then enter the dimensions for your folded dipole antenna. If you make the diameter of both radiator elements, d1 and d2, equal the transformation ratio will be 4. This should transform the 72 Ohm simple dipole feed impedance to about 288 Ohms. You should note that, when the two diameters are equal, the distance s does not change the transformation ratio. Use the text areas below to enter diameters of each element and the distance between them. You can enter the the data in any dimension you like. You can even mix and match. Output data is presented in both US/Imperial and Metric dimensions.